Sion's got way too many mechanics working against him to be good.
1). Remove the gong noise from his ult. There's already a HUGE amount of counter-play to his ult. The gong noise is absolutely unacceptable considering how the ultimate works.
2). Sion's Q is incredibly impractical to use. The thing takes too long to charge to get the full damage and CC out of it, and can be interrupted too easily, and the range and width of the spell is far too small. Even the 1 second version of the spell doesn't do enough damage.
3). Why is Sion's base health so small when his HP-scaling on his Soul Furnace is really abysmal? This makes no sense. Why give a champion scaling if you're just going to gut the base stats? This is why Cassiopeia is failing. The exact reason.
4). I would like to see Sion's AP Ratio's increase WITHOUT NERFING ANYTHING ELSE ON IT.
Sion's very fun to play. But he's not very effective.